The present invention relates to modified rubber concentrates formed (at least in part) through emulsion polymerization or emulsion blending. Grafted rubber concentrates are one type of modified rubber concentrate. They are typically formed by emulsion polymerization, where a monomer is, or monomers are, grafted through polymerization onto colloidally suspended rubber particles.
The thus polymerized particles are then coagulated. Chemical or shear coagulation are the typical commercially used coagulation techniques. The coagulated polymer particles are separated from the emulsion carrier by pressing, centrifuging or the like. A rinse step may be employed to rinse the coagulated polymer.
Another type of modified rubber concentrate is formed by blending emulsions of rubbers and graft rubber compounds. Again, coagulation must be effected and the resulting coagulated polymer blend de-watered.
In either case, de-watering is typically a problem. Normal 40 to 50% rubber ABS emulsion, when freeze coagulated, produces a fine powder which is extremely difficult to de-water. Pure rubber emulsions when coagulated produce a sponge-like structure which is extremely difficult to de-water.
These de-watering difficulties are encountered over a wide range of emulsion polymerized or blended modified rubber concentrates regardless of the de-watering technique used (centrifuging or mechanical pressing being typical). As a result, energy has to be expended to further dry the rubber concentrate before it can be used.
U.S. Pat. No. 4,401,806 to Hagiwara et al. entitled PROCESS FOR PRODUCING GRAFT COPOLYMER discloses a method for improving the powder characteristics of graft copolymers by mixing the copolymer latex with a particular organic solvent immediately after or simultaneously with the coagulation of the latex, while maintaining a dispersion in which water forms a continuous phase. The problem with this approach is that one has to then separate the organic solvent from the end product rubber as well as separating water from the end product rubber. A steam stripping step or the like would have to be used to insure that all solvent is removed. These rubbers are commonly used to reinforce styrene, which is used in refrigerators and in food packaging. One does not want solvent to be migrating out of the plastic.